Stable isotope tracer methods for in vivo investigations.

During the last two decades, in parallel with the growth of modern electronics, several new techniques have been developed for measuring stable isotopic enrichments in biochemistry and medicine. The development and potential of these techniques are discussed. Of these methods, mass spectrometry has been developed and refined the fullest to quantitate stable isotope tracers in very minute samples and for very large dilutions of tracer. No single mass spectrometric technique can measure this entire range, and different techniques are used for different applications. Examples of the different methods are presented for determining whole-body amino acid and protein dynamics in humans with stable isotopically labeled amino acid tracers.